Method and inlet control system for controlling a gas flow sample to an evacuated chamber

ABSTRACT

A method and inlet control system for controlling a gas flow sample to an evacuated chamber such as found in a mass spectrometer, is disclosed. The system utilizes a short inlet passage having an effective opening determined by a tapered diamond or steel tip needle adjacent to the inlet passage. The needle is positionally adjusted with respect to the inlet passage by being mounted on a piezoelectric crystal which is flexed by coupling thereto an electric potential derived by sensing the ions at the ionization chamber of a mass spectrometer, for example, and developing therefrom an electric signal indicative of the total pressure within the ionization chamber. The signal coupled to the piezoelectric crystal is preferably a pulse-width modulated signal with the needle maintaining the inlet passage closed except during the time that the piezoelectric crystal is flexed due to a received pulse. A vacuum pump, and a quadrupole filter, both of which are relatively small, are also disclosed, so that a mass spectrometer system, for example, is sufficiently compact so as to be useful, in conjunction with a respiratory valve, for the analysis of respiratory gases. The method for controlling a gas sample flow to a mass spectrometer, for example, comprises providing an inlet for a gas sample into the ionization chamber of the mass spectrometer, monitoring the pressure within the ionization chamber and developing an electrical signal indicative thereof, and utilizing the developed electrical signal to control the flow of gas sample through the inlet to maintain a substantially constant pressure within the ionization chamber.

United States Patent [1 1 Sodal et al.

University of Colorado, Boulder, C010.

[22] Filed: Apr. 30, 1973 [21] Appl. No.: 355,792

[73] Assignee:

[52] US. Cl. 250/288; 250/282; 250/457 [51] Int. Cl. H01] 39/34 [58]Field of Search 250/288, 289, 457, 282

[56] References Cited UNITED STATES PATENTS 2,601,097 6/1952 Crawford250/288 2,721,270 10/1955 Bennett 250/288 2,769,912 11/1956 Lupfer etal. 250/288 3,458,699 7/1969 Padrta 250/288 3,500,040 3/1970 Padrta250/288 3,675,072 7/1972 Hahn et al. 250/457 OTHER PUBLICATIONSPiezoelectric Transducers Berlincourt et a1. Electro- Tech. pp. 33-44January, 1970.

BIMORPH CRYSTAL DIAMOND TIP NEEDLE-- arse AVAILABLE 66W [451 July 15,1975 [5 7] ABSTRACT A method and inlet control system for controlling agas flow sample to an evacuated chamber such as found in a massspectrometer, is disclosed. The system utilizes a short inlet passagehaving an effective opening determined by a tapered diamond or steel tipneedle adjacent to the inlet passage. The needle is positionallyadjusted with respect to the inlet passage by being mounted on apiezoelectric crystal which is flexed by coupling thereto an electricpotential derived by sensing the ions at the ionization chamber of amass spectrometer, for example, and developing therefrom an electricsignal indicative of the total pressure within the ionization chamber.The signal coupled to the piezoelectric crystal is preferably apulse-width modulated signal with the needle maintaining the inletpassage closed except during the time that the piezoelectric crystal isflexed due to a received pulse. A vacuum pump, and a quadrupole filter,both of which are relatively small, are also disclosed, so that a massspectrometer system, for example, is sufficiently compact so as to beuseful, in conjunction with a respiratory valve, for the analysis ofrespiratory gases. The method for controlling a gas sample flow to amass spectrometer, for example, comprises providing an inlet for a gassample into the ionization chamber of the mass spectrometer, monitoringthe pressure within the ionization chamber and developing an electricalsignal indicative thereof, and utilizing the developed electrical signalto control the flow of gas sample through the inlet to maintain a substantially constant pressure within the ionization chamber.

12 Claims, 4 Drawing Figures I I I I I I 100 volts ------0 8 PULSE-WIDTHMOOULATED PK CONTROL SIGNAL SAMPLE GAS l/(ONTROL AMF.

FILAMENT TO VACUUM PUMP (42) 7MAss FILTER MULTIPLIER I) so SIGNALP'A'TENTEDJUL 1 m5 3.895231 v 'Pr-Q sulll PRESS o PRESS 21 0 TORR I TIME(sec) FIG. 4

METHOD AND INLET CONTROL SYSTEM FOR CONTROLLING A GAS FLOW SAMPLE TO ANEVACUATED CHAMBER The invention described herein was made in the courseof work under a grant or award from the Department of Health, Education,and Welfare.

FIELD OF THE INVENTION This invention relates to a method and inletcontrol system for controlling a gas flow sample to an evacuated chamberincluding such evacuated chambers as are found in a mass spectrometerand a sputtering system.

BACKGROUND OF THE INVENTION Much of pulmonary physiology is based on theanalysis of respiratory gases. Because of its potential as a high speedaccurate gas analyzer, the mass spectrome; ter has attractedconsiderable attention in this field. However, the instrument has failedto reach its potential at least in part due to the necessity for a longcapillary inlet system which can, and often does, destroy the integrityof the gas sample and causes instability in the instrument.

Thus, while mass spectrometers have been available to respiratoryphysiologists for about 20 years, they have not achieved the widespreadapplication that was once predicted. With respect to the technicalshortcomings in spectrometer design at least for pulmonary physiologypurposes, the sample inlet system is one of the major problems.

Inherent with mass spectrometry as well as with a sputtering system isthat an immense pressure difference exists between the site at which gasis sampled and the inside of the spectrometer. Traditionally thispressure drop is achieved in two stages. Firstly, a long slendersampling capillary tube is used which produces the major fall inpressure. Secondly, at the end of the capillary a fixed molecular leakis employed to achieve the final pressure drop.

The capillary is required because the size of known spectrometers doesnot permit them to be brought into close proximity to the source ofsample gas such as a respiratory valve. (See Fowler, K. T., TheRespiratory Mass Spectrometer, PHYSICS IN MEDICINE AND BIOLOGY, Volume14, pages 185-199, 1969.) This arrangement has several adverse effectson instrument performance.' Firstly, there are distortions introduced bythe behavior of water vapor. During the respiratory cycle sample gasswings between dry inspired and wet expired gas. Water vapor traverses aheated sampling capillary about times more slowly than the otherrespiratory gases. Hence, the ionizer sees a fluctuating water vaporlevel which does not reflect the pressure of water vapor at the frontend of the capillary. Unpredictable errors in precision occur becausethe dilution effect due to water vapor is not the same as existed at themouth. At an oxygen tension of 100 mm Hg this error could be as great as8% if no correction is applied. Various methods for correction of thisproblem have been employed, but only to obtain a more accuraterelationship between the gases of greatest interest. (See Scheid, P.,Slama, H., and Piiper, 1., Electronic Compensation of the Effects ofWater Vapor in Respiratory Mass Spectrometry, J. APPL. PHYSIOL, Volume30, pages 258-260, 1971). Secondly, the sampling capillary introducesdelay in response and deterioration of rise time of the instrument.Although this could theoretically be measured and corrected for, smallvariations in pumping speed cause relatively large changes in transittime such that in practice it is difficult to achieve this correctionaccurately. This creates problems when data concerning gas concentrationare to be combined with other information such as gas flow rates as inthe measurement of oxygen uptake. Lastly, even though the geometry ofthe sample conduit and inlet are fixed the actual rate of molecular flowinto the spectrometer tends to vary from moment to moment becausefactors such as particle deposition and changes in gas composition alterthe conductance of the inlet system.

Since the mass spectrometer is a particle counting device variations inmolecular leak rate due to the above factors constitute a source ofrandom error. Hence, it is apparent that the way by which the gas sampleis introduced into the ionizer is the most critical step in themeasurement of respiratory gases by a mass spectrometer. A more accuratemeasurement of the sample line would be made short and the volume of theconduits in front of the ionizer and the ionization chamber madesmaller.

SUMMARY OF THE INVENTION This invention provides an evacuated chamber,such as is found in a mass spectrometer system, that does not require alengthy capillary inlet tube. The inlet system of this inventionincludes valve means adjacent to a small orifice providing an inletpassage into the evacuated chamber of, for example, a mass spectrometer,with the valve means being positionally controlled by valve positioncontrol means that may be made responsive to an electrical signalderived by monitoring the pressure within the evacuated chamber. Byutilizing the foregoing, system stability is improved and accuratemeasurement of all respiratory gases, including water vapor, isfacilitated. In addition, by reducing component size, the overall systemis made sufficiently compact so as to be particularly useful for directattachment to a respiratory valve.

It is therefore an object of this invention to provide a new and novelmethod and inlet control system for controlling a gas flow sample to anevacuated chamber.

It is another object of this invention to provide an improved massspectrometer system that is compact yet provides good stability andaccurate measurements.

It is another object of this invention to provide an improved massspectrometer having a new and novel inlet system.

It is still another object of this invention to provide an inlet systemfor an evacuated chamber that does not require a lengthy capillary inlettube.

It is yet another object of this invention to provide an inlet systemfor an evacuated chamber, including that used in a mass spectrometer,that includes a valve means and valve position control means.

It is another object of this invention to provide a servo-controlledinlet system for an evacuated chamber that automatically maintains thetotal pressure within said chamber at a predetermined level.

It is yet another object of this invention to provide a unique methodfor controlling sample gas flow to a mass spectrometer.

It is still another object of this invention to provide an inlet controlsystem for a mass spectrometer that has a low flow capability withoutadversely affecting good system stability.

With these and other objects in view. which will become apparent to oneskilled in the art as the description proceeds, the invention resides inthe novel construction, combination, and arrangement of partssubstantially as hereinafter described, and more particu larly definedby the appended claims, it being understood that such changes in theprecise embodiment of the herein disclosed invention are meant to beincluded as come within the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate acomplete embodiment of the invention according to the best mode so fardevised for the practical application of the principles thereof, and inwhich:

FIG. 1 is a simplified schematic representation of the mass spectrometersystem of this invention including a servo-controlled inlet system;

FIG. 2 is a simplified schematic representation of a mass spectrometeras shown in FIG. 1 but showing the system attached to a respiratoryvalve;

FIG. 3 is an illustration of a typical cycle during normal operation ofthe system of this invention; and

FIG. 4 is an illustration of a typical cycle when the inlet system ofthis invention is not utilized.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings inwhich like numerals have been used for like characters, the numeral 7refers generally to the mass spectrometer of this invention. As shown inFIGS. 1 and 2, the mass spectrometer system includes an inlet passage 8through which sample gas is introduced into ionization chamber 9. Theinlet passage 8 is formed in the top portion of the spectrometer housing11, which housing may be formed by a thin stainless steel membrane (6mills) with a small hole (2 mills) 12 which can be occluded to a varyingdegree by a diamond-tipped or steel-tipped needle 13. The needle ismounted on a piezoelectric crystal 14 (at the end of body portions 15 ofneedle 13 opposite the tapered diamond or steel tip 16 which crystal hasthe property of flexing when an electric potential is applied to it(described more fully, for example, in Berlincourt, D.A., PiezoelectricTransducers, ELECTRO- TECHNOLOGY, pages 33-44, January, 1970). Themovement of the needle and hence the leak rate is a function of thevoltage applied to the crystal.

A signal proportional to the total pressure in the ionizer (as broughtout more fully hereinafter) provides the input to a control amplifier,or signal processing means, 18 through lead 19 to drive the crystal,permitting a servo-controlled movement of the needle so as to maintainionizer pressure constant. This servo-system controlling the leak (i.e.,introduction of sample gas into the ionization chamber of the massspectrometer) has an extremely rapid response time (T 2 msec) and it iscapable of operating the leak to compensate for the fastest changes ingas composition, water vapor effects, etc., which in existing systemschange the ionization pressure and thereby introduce errors in thesignal output from the.mass spectrometer. As shown schematically (at 20)in FIG. 1, a pulse-width modulated, pressure-controlled signal coupledto the crystal from control amplifier 18 through lead 22 provides a lowflow inlet, but at the same time permits a large inlet orificepreventing clogging of the lead and separation of gases. With the shortdistance between the leak and the ionizer both the delay time and theresponse time are greatly reduced compared to that of a capillary inletsystem. Positioning the crystal (which crystal is conventionallymaintained inposition in mount 26, for example) such that when novoltage is applied, the mechanical stress on the crystal is sufficientto close the leak, as indicated in the dotted lines of FIG. 1. Hence,the leak will close automatically if the instrument is in a standbymode, or more important, the leak will always be closed if power isremoved. This essentially means that the mass spectrometer system isfail safe, and that it can be moved from one location to another withoutfirst going through a complex shut-down procedure. If a high qualityvacuum system is employed, it is conceivable that the system couldmaintain its vacuum over several days without pumping or any form ofpower input. In a working embodiment of this invention, the leak hasbeen tested in a closed position with a helium leak detector and nomeasurable leakage was detected. This test was conducted after the leakhad been in continuous operation for 3 weeks in a laboratory atmosphereand occasionally exposed to expired respiratory gas during this period.The same leak was also tested for its mechanical stability. After beingdropped from several inches down to a table top, the leak was unchanged.

In order to decrease instrument response time without increasing pumpingrequirements, a new smaller and more efficient ionizer is utilized inthis invention. The ionizer has built-in pressure measuring capabilitywhich is used to control the leak rate and for calculation of gastensions. This is achieved by using a small ionization chamber 9 oflessthan 2 cc total volume (see FIG. 1 which conducts a high flow rate ofsample gas directly from the leak, thus providing a fast response timefor the system. A conventional filament 30 is utilized (electricalconnections are not shown for simplicity) in the ionization chamber. Theion beam is emitted from the chamber through aperture 31. Two plates 33and 34 provide electrostatic focusing of the ion beam and a third plate36 with a smaller orifice 37 picks off part of the ion beam and suppliesthe signal to electrometer 18 for pressure monitoring. The mainadvantage in this unique way of measuring ionizer pressure is that itgives an instant and accurate representation of ionizer pressure as wellas measuring the ion beam which actually enters the mass filter 40 thususing the signal which most directly affects the output of the massspectrometer as a control signal for the leak.

As shown in FIGS. 1 and 2, the sample gas passing through passage 8 isintroduced into ionization chamber 9, and a vacuum pump 42 also isconnected with the ionization chamber, as is common for massspectrometers, through passage 44. After the ion beam is emitted fromionization chamber 9, focused by plates 33 and 34, and a portion pickedoff by plate 36, the beam is directed to a mass filter 40, which asindicated in FIG. 2, can be conventional quadrupole filter (such filtersare discussed, for example, in Pedan, J., The Quadrupole Approach,INDUSTRIAL RESEARCH, pages 50-52, April, 1970; and Wiesendanger, H. U.D., Quadrupole Mass Spectrometry, AMERICAN LAB- ORATORY, pages 35-43,July, 1970). At the outlet of the mass filter, the beam isconventionally directed through multiplier 48 to plate 50 where theoutlet signal is developed and coupled from the system through lead 52.As indicated in FIG. 2, this signal, along with a signal from plate 36on lead 19, may be coupled to a computer (not shown) for conventionalprocessing.

As shown in FIG. 2, a respiratory valve 56 can be provided. The passage8 preferably communicates with the middle chamber of such a valve sothat both inspiratory and expiratory gas can be sampled by the massspectrometer. As shown in FIG. 2, the gas from a subject is introducedinto the respiratory valve through tube 58, and a micrometer 60 may alsobe provided. The system of this invention has been built and testedwith'a quadrupole filter. FIG. 3 shows the output of the instrument whentuned to measure oxygen during an expiratory breathing cycle by blowingacross the leak assembly such that a large amount of water vapor andparticles in the expired air were deposited on the leak. No specialmouth piece or tube was attached to the system and the figure is meantto serve only as an illustration where the servo-control is operational.The chamber pressure was monitored and displayed below the O tracingusing an ionization gauge. The small change in chamber pressure overthis period caused a change of less than 0.5% in the oxygen signal.Since the ionization gauge is also affected by the changing gasconcentration (decreased 0 and increased CO in the chamber, anevaluation of the accuracy should be based on the output signal for eachgas from the mass spectrometer. If the servo-control on the leak isdisabled, the leak clogs up very rapidly as indicated in FIG. 4. Here,the system was exposed to a short burst of expiratory gas (approx. 1sec.) and the chamber pressure changed several fold. The change inoxygen signal in this case is mainly due to pressure change in theionizer. Even when exposed to room air only, the leak would clog up veryrapidly from dust particles in the air.

Thus, in operation the mass spectrometer of this invention receives gasthrough chamber 8 and the amount of gas introduced into the ionizationchamber 9 is controlled by a servo-control system which includes apiezoelectric crystal (indicated as a bimorph crystal in FIGS. 1 and 2)that flexes due to application of an electric potential. The electricpotential is generated by a sensing plate 36 in the path of the ion beamwith the electrical output signal from plate 36 being coupled to controlamplifier, or signal processing means, 18. As shown in FIG. 1, theoutput to the piezoelectric crystal is preferably a pulse-widthmodulated signal, such as indicated at 20. Such a signal isconventionally formed and not detailed herein, but rather onlyindicated. In like manner, the typical operation of a mass spectrometer,as well as other details have been left out of this description forsimplicity.

In addition to the reference set out hereinabove, the following may beconsulted for further systems and/or component details: Abrahamsson, S.,The Use of Computers In Low Resolution Mass Spectrometry, SCIENCE TOOLS,Volume 14, pages 29-34, 1967; Beckman Instruments, Inc., MetabolicActivity Gas Analyzer, Technical Report; Brubaker, W. M., A Study of theIntroduction of Ions into the Region of Strong Fields Within Aquadrupole Mass Spectrometer, Final Report NASA-CR-9l801, August,1965-October, 1967; Brubacker, W. M. Theoretical and ExperimentalComparisons of Quadrupole Mass Analyzers with Round and HyperbolicField-forming Surfaces," Invited Paper, International Conference on MassSpectrometry, September, 1969, Kyoto, Japan; Dardik, H., and Laufman,H., On-line In Vivo Measurements of Partial Pressure of Oxygen andCarbon Dioxide of Blood, Tissue, and Respired Air by Mass Spectrometry,SURG. GYN. & OBSTET., Volume 131, pages 1157-1160, 1970; Dawson, P. H.Hedman, J. S.. and Whetten, N. R., A Simple Mass Spectrometer. THEREVIEW OF SCIENTIFIC INSTRUMENTS. Volume 40(11), pages 1444-1450,November 1969, Jones, W. B., Finchum, R. N., Russell R. O. Jr., andReeves, T. J., Transient Cardiac Output Response to Multiple Levels ofSupine Exercise," J. APPL. PHYSIOL, Volume 28, pages 183-189, 1970;Jones, W. B., Reeves, T. J., Total Cardiac Output Response During FourMinutes of Exercise, AMER, HEART J., Volume 76, pages 209-216, 1968; andKim, T. S., Rahn, H. and Farhi, L. E., Estimation of True Venous andArterial P by Gas Analysis of a Single Breath," J. APPL. PHYSIOL, Volume21, pages 1338-1344. 1966.

Although the above description relates to the use of a method and inletcontrol system for controlling a gas flow sample to a mass spectrometerand a novel mass spectrometer, it will be understood that this inventionis not so limited and may be used in controlling pressures in vacuumchambers such as, for example, those used in a sputtering system.Accordingly, it will now be appreciated that this invention relates toan inlet control system for an evacuated chamber in which said systemcomprises an inlet passage opening into the interior of said evacuatedchamber to introduce gas samples therethrough, valve means at said inletpassage for controlling the effective opening through said passage, andvalve control means for controlling the positioning of said valve meansthereby to control the introduction of gas samples to said evacuatedchamber. The evacuated chamber into which the gas samples are introducedmay be constructed with a relatively small volume, i.e., less than about2 cc total volume. The valve means may be made to be responsive to thepressure within the evacuated chamber thereby controlling the openingthrough the inlet passage to maintain a substantially constant pressurewithin the evacuated chamber. Further, this invention relates to amethod for controlling gas flow sample to an evacuated chambercomprising providing an inlet for gas sample into the evacuated chamber,sensing or monitoring the pressure within the evacuated chamber anddeveloping or generating an electrical signal indicative thereof, andutilizing the generated or developed signal to control the flow of gassample through the inlet to maintain a substantially constant pressurewithin the evacuated chamber. The method also includes providing arespiratory valve from which gas sample is taken through said inlet sothat both inspired and expired gas samples may be tested.

From the foregoing, it can be seen that this invention provides a newand novel method and inlet system for an evacuated chamber, as well as anew and novel mass spectrometer.

What is claimed is:

l. A servo-controlled inlet system for a mass spectrometer, said systemcomprising: an inlet passage opening into the interior of said massspectrometer and through which gas may be introduced into said massspectrometer; valve means at said inlet passage for confor sensing thepressure within said spectrometer and developing an electrical signalindicative thereof; and signal processing means for receiving saidsignal from said sensing means and responsive thereto controlling saidvalve positioning means whereby said valve means is automaticallyadjusted in position dependent upon sensed pressure.

2. The system of claim 1 wherein said inletpassage opens into theionization chamber of said mass spectrometer. and wherein said sensingmeansincludes an ion sensitive plate for intercepting a portion of theions developed within said ionization chamber.

3. The system of claim 1 wherein said signal processing means includescontrol amplifier means for receiving the output from said sensing meansand developing a signal suitable for controlling said valve positioningmeans.

4. The system of claim 3 wherein said control amplifler means produces apulse-width modulated control signal that is coupled to said valvepositioning means to control the same.

5. A servo-controlled inlet system for a mass spectrometer said systemcomprising: an inlet passage. opening into the ionization chamber ofsaid mass spectrometer and through which gas may be introduced thereto;valve means including a needle valve one end of which is tapered andpositionally adjustable adjacent to said inlet passage to control theeffective opening thereof and thereby control the amount of gas flow insaid inlet passage; a piezoelectric crystal; means connecting saidpiezoelectric crystal with said needle valve so that the position ofsaid valve is controlled by the amount of flexing of said crystal;sensing means within the spectrometer for developing an electric signalby ion sensing that is indicative of the pressure within said ionizationchamber; and signal processing means connected to receive saidelectrical signal from said sensing means and producing an output thatis coupled to said piezoelectric crystal to control the amount offlexingof said crystal.

6. The system of claim 5 wherein said piezoelectric crystal maintainssaid needle valve in a position such that said inlet passage is closedexcept when a predetermined signal is received from said signalprocessing means to flex said crystal in a manner so as to open saidvalve. I

7. The system of claim 6 said signal processing means produces apulse-width modulated signal that is coupled to said piezoelectriccrystal so that said crystal maintains said inlet passage closed exceptduring the occurrence of each pulse of said pulse-width modulatedsignal.

8. A mass spectrometer system comprising: ion prov8 ducing and handlingmeans including an ionization chamber; gas sampling means including aninlet passage opening into said ionization chamber to conduct gassamples thereto, said gas sampling means including a respiratory valveone portion of which communicates with said inlet passage; valve meansfor determining the effective opening through said inlet passage;sensing means for sensing the pressure within said ionization chamberand developing an electrical signal indicative thereof; and controlmeans including a piezoelectric crystal connected with said valve means,said control means receiving said signal from said sensing means andresponsive thereto controlling the flexing of said piezoelectric crystalto thereby positionally control said valve means dependent upon sensedpressure.

9. A mass spectrometer system comprising: a respiratory valve; anionization chamber; a vacuum pump connected with said ionizationchamber; an inlet passage opening into said ionization chamber from saidrespiratory valve and through which gas may be introduced into saidionization chamber; valve means including a needle valve one end ofwhich is tapered and positionally adjustable adjacent to said inletpassage to control the effective opening thereof; a piezoelectriccrystal; means connecting said piezoelectric crystal with said needlevalve so that positioning of said valve is controlled by the amount offlexing of said crystal; focusing plates to focus ions emitted from saidionization chamber; a sensing plate adjacent to said focusing plates toreceive a portion of ions emitted from said ionization chamber; aquadrupole filter through which said emitted ions are directed by saidfocusing plates; and signal processing means connected with said sensingplate and with said piezoelectric'crystal whereby a signal developed bysaid'sensing plate is utilized to control the amount of flexing of saidpiezoelectric crystal.

10. The system of claim 9 wherein said elements are relatively small sothat said system is sufficiently compact 'to enable close usage andenhance system stability.

11. A method for controlling gas sample flow to a mass spectrometer,said method comprising: providing an inlet for gas sample into theionization chamber of a mass spectrometer, said inlet having an openingthe size of which is dependent upon the amount of flexing of apiezoelectric crystal; monitoring the pressure within the ionizationchamber by ion sensing and developing an electrical signal indicativethereof; and utilizing the developed electrical signal to control theflexing of said piezoelectric crystal to thereby control the flow of gassample through the inlet to maintain a substantially constant pressurewithin the ionization chamber.

12. The method of claim 11 further including providing a respiratoryvalve from which gas sample is taken through said inlet so that bothinspired and expired gas 7 samples may be tested by themass'spectrometer.

1. A servo-controlled inlet system for a mass spectrometer, said systemcomprising: an inlet passage opening into the interior of said massspectrometer and through which gas may be introduced into said massspectrometer; valve means at said inlet passage for controlling theeffective opening through which gas may be introduced into said passage;valve positioning means including a piezoelectric crystal connected withsaid valve means whereby said valve means is positionally controlled bythe flexing of said crystal; sensing means for sensing the pressurewithin said spectrometer and developing an electrical signal indicativethereof; and signal processing means for receiving said signal from saidsensing means and responsive thereto controlling said valve poSitioningmeans whereby said valve means is automatically adjusted in positiondependent upon sensed pressure.
 2. The system of claim 1 wherein saidinlet passage opens into the ionization chamber of said massspectrometer, and wherein said sensing means includes an ion sensitiveplate for intercepting a portion of the ions developed within saidionization chamber.
 3. The system of claim 1 wherein said signalprocessing means includes control amplifier means for receiving theoutput from said sensing means and developing a signal suitable forcontrolling said valve positioning means.
 4. The system of claim 3wherein said control amplifier means produces a pulse-width modulatedcontrol signal that is coupled to said valve positioning means tocontrol the same.
 5. A servo-controlled inlet system for a massspectrometer said system comprising: an inlet passage opening into theionization chamber of said mass spectrometer and through which gas maybe introduced thereto; valve means including a needle valve one end ofwhich is tapered and positionally adjustable adjacent to said inletpassage to control the effective opening thereof and thereby control theamount of gas flow in said inlet passage; a piezoelectric crystal; meansconnecting said piezoelectric crystal with said needle valve so that theposition of said valve is controlled by the amount of flexing of saidcrystal; sensing means within the spectrometer for developing anelectric signal by ion sensing that is indicative of the pressure withinsaid ionization chamber; and signal processing means connected toreceive said electrical signal from said sensing means and producing anoutput that is coupled to said piezoelectric crystal to control theamount of flexing of said crystal.
 6. The system of claim 5 wherein saidpiezoelectric crystal maintains said needle valve in a position suchthat said inlet passage is closed except when a predetermined signal isreceived from said signal processing means to flex said crystal in amanner so as to open said valve.
 7. The system of claim 6 said signalprocessing means produces a pulse-width modulated signal that is coupledto said piezoelectric crystal so that said crystal maintains said inletpassage closed except during the occurrence of each pulse of saidpulse-width modulated signal.
 8. A mass spectrometer system comprising:ion producing and handling means including an ionization chamber; gassampling means including an inlet passage opening into said ionizationchamber to conduct gas samples thereto, said gas sampling meansincluding a respiratory valve one portion of which communicates withsaid inlet passage; valve means for determining the effective openingthrough said inlet passage; sensing means for sensing the pressurewithin said ionization chamber and developing an electrical signalindicative thereof; and control means including a piezoelectric crystalconnected with said valve means, said control means receiving saidsignal from said sensing means and responsive thereto controlling theflexing of said piezoelectric crystal to thereby positionally controlsaid valve means dependent upon sensed pressure.
 9. A mass spectrometersystem comprising: a respiratory valve; an ionization chamber; a vacuumpump connected with said ionization chamber; an inlet passage openinginto said ionization chamber from said respiratory valve and throughwhich gas may be introduced into said ionization chamber; valve meansincluding a needle valve one end of which is tapered and positionallyadjustable adjacent to said inlet passage to control the effectiveopening thereof; a piezoelectric crystal; means connecting saidpiezoelectric crystal with said needle valve so that positioning of saidvalve is controlled by the amount of flexing of said crystal; focusingplates to focus ions emitted from said ionization chamber; a sensingplate adjacent to said focusing plates to receive a portion of ionsemitted from said ionization chamber; a quadrupole filter through wHichsaid emitted ions are directed by said focusing plates; and signalprocessing means connected with said sensing plate and with saidpiezoelectric crystal whereby a signal developed by said sensing plateis utilized to control the amount of flexing of said piezoelectriccrystal.
 10. The system of claim 9 wherein said elements are relativelysmall so that said system is sufficiently compact to enable close usageand enhance system stability.
 11. A method for controlling gas sampleflow to a mass spectrometer, said method comprising: providing an inletfor gas sample into the ionization chamber of a mass spectrometer, saidinlet having an opening the size of which is dependent upon the amountof flexing of a piezoelectric crystal; monitoring the pressure withinthe ionization chamber by ion sensing and developing an electricalsignal indicative thereof; and utilizing the developed electrical signalto control the flexing of said piezoelectric crystal to thereby controlthe flow of gas sample through the inlet to maintain a substantiallyconstant pressure within the ionization chamber.
 12. The method of claim11 further including providing a respiratory valve from which gas sampleis taken through said inlet so that both inspired and expired gassamples may be tested by the mass spectrometer.